The invention is based on a printing chip for a printing head working according to the ink-jet printing principle, in accordance with the preamble of claim 1.
DE 44 43 254 C1 discloses a printing chip of the generic type. This printing chip has a medium chamber formed by a recess in a glass plate, which is also referred to as a support plate. The recess is made to such a depth in the glass plate that the bottom of the recess serves as a deflectable diaphragm, which thus forms one wall of the medium chamber. A duct for feeding an ink that is to be squirted out opens in the medium chamber. Furthermore, a squirting-out opening is provided, which is connected to the medium chamber. The recesses introduced in the glass plate are closed off by a central plate. The known printing chip is thus formed from two substrate parts lying one on top of the other, at least one substrate part being composed of glass.
This known printing chip has the disadvantage that it is not suitable, for lack of thermal stability, for squirting out very hot media at temperatures above 500xc2x0 C.
The object of the invention, therefore, is to specify a printing chip of the type mentioned in the introduction which does not have this disadvantage.
This object is achieved by means of a printing chip for a printing head working according to the ink-jet printing principle, the printing chip having the features of claim 1. It thus has at least one medium chamber formed by a recess in the printing chip. Furthermore, a deflectable diaphragm is provided, which forms one wall of the medium chamber. Moreover, a duct for feeding a liquid medium that is to be squirted out hot opens in the medium chamber. In addition, the printing chip has a squirting-out opening connected to the medium chamber. According to the invention, the printing chip is distinguished by the fact that it is produced exclusively from monocrystalline silicon. The printing chip according to the invention is distinguished by high operational reliability when media at temperatures reaching in excess of 1000xc2x0 C. are sprayed. The known printing chips comprising glass cannot be used to spray such hot media since the glass softens at these temperatures, so that the diaphragm is damaged and/or can no longer be deflected in a reproducible manner, as a result of which an exact drop volume can no longer be ejected from the squirting-out opening. The printing chip according to the invention is also distinguished by the fact that the monocrystalline silicon can be processed according to known and proven fabrication processes in order to be able to produce the medium chamber and the remaining ducts and/or the squirting-out opening.
One development of the invention provides for the printing chip to be formed from at least two substrate parts lying one on top of the other. This affords the advantage for production that the required recesses for the medium chamber and the ducts and also the squirting-out opening can be introduced from one side on the substrate parts. The recesses are then closed off by the substrate parts being laid one on top of the other. The printing chip comprising two substrate parts lying one on top of the other is thus simple and cost-effective to produce.
According to a particularly preferred exemplary embodiment, the feeding duct, the medium chamber and the squirting-out opening are arranged in the printing chip in such a way that the medium to be squirted out flows alternately through the substrate parts. What is advantageous in this case is that it is possible to produce the recesses at least for the medium chamber, the feeding duct and squirting-out opening in the two substrate parts using different fabrication processes. Thus, by way of example, one of the substrate parts can be processed by anisotropic wet etching and the other substrate part by dry etching or another material-removing process, in order to introduce the recesses. By rotating the (100) silicon crystal plane of one of the substrate parts by 45xc2x0 relative to the other substrate part, it is possible for all of the recesses to be formed favourably in terms of flow properties in the course of the anisotropic wet etching. Since both substrate parts are produced from silicon, which has very good thermal conductivity, strain nevertheless does not arise in the printing chip on account of thermal expansion.
A preferred exemplary embodiment provides for a medium supply chamber, to which the feeding duct is connected, to be located in the printing chip. It is thus possible to provide a large-scale integrated printing chip which contains all the essential functional units. If appropriate, however, it would also be conceivable to design the medium supply chamber, which can also be referred to as a storage chamber, in a separate structural part which can then be fixed to the printing chip. However, if the medium supply chamber is realized in the printing chip, all functionally essential parts can be realized by recesses that are simple to produce in the substrate parts.
A preferred exemplary embodiment provides for the printing chip surfaces, in particular those which come into contact with the hot medium, to be provided with a coating that is resistant to high temperatures. Thus, the hot liquid medium cannot damage the printing chip substrate even over a long service life of the printing chip.
It is preferably provided that the printing chip surfaces, in particular those which come into contact with the hot medium, are passivated by thermal oxide. The thermal oxide is preferably provided on the channel walls and the wall of the medium chamber. In particular, then, the printing chip surfaces past which the hot medium flows are passivated with thermal oxide.
In a particularly preferred exemplary embodiment, the two substrate parts are connected to one another inseparably by silicon fusion bonding. This connection method, which can only be employed for silicon-silicon, ensures reliable connection of the two substrate parts even at operating temperatures of far in excess of 1000xc2x0 C. Consequently, by comparison with known printing chips, the printing chip is also distinguished by temperature-resistant connection of the two substrate parts to one another.
According to a preferred exemplary embodiment, the recess for the medium chamber, the feeding duct, the squirting-out opening and the medium supply chamber are produced by an anisotropic wet or dry etching process, in particular anisotropic wet etching using potassium hydroxide (KOH) being preferred. It goes without saying that other basic etching solutions, in particular alkaline metal hydroxide solutions, can be used. Anisotropic etching in monocrystalline silicon with (100) orientation produces the recesses for the ducts and the medium chamber always with four pyramidally inclined side walls on the (111) crystal planes. Their angle with respect to the (100) crystal plane of the substrate surface of the silicon is 54.7xc2x0 on all sides. The dimensions of the recesses can be produced very accurately in this etching process. This affords the advantage, in particular in the substrate part having the recess for the medium chamber, that the diaphragm area can be produced very accurately, if the said diaphragm area is formed by the bottom of the recess. The diaphragm area is thus merely dependent on the opening width of the etching mask and the depth to be etched for the recess. Since the angle of the side walls always has the same value, the effective diaphragm area can thus be produced particularly accurately in a manner dependent on the opening width of the perforated mask and the depth of the recess. If a plurality of medium chambers each having a deflectable diaphragm are provided, all of the diaphragms have essentially the same area in this production process, so that, given the same deflection excursion of the diaphragm, the same drop size can be ejected from each chamber. In particular, the printing chip is thus distinguished by highly accurate drop volumes that can be squirted out.
In a preferred embodiment, the medium chamber, the feeding duct and the squirting-out opening havexe2x80x94as seen in the flow direction of the mediumxe2x80x94a trapezoidal or V-shaped cross section. By the choice of monocrystalline (100) silicon, these cross sections can be produced very accurately sincexe2x80x94as mentioned abovexe2x80x94the side walls always form an angle of 54.7xc2x0 with the (100) crystal plane.
Preferably, it is also provided that the medium chamber is of rectangular design in plan view, and that the feeding duct opens in the medium chamber, and the squirting-out opening is connected to the medium chamber, in such a way that the medium flows through the said medium chamber essentially diagonally. The effect that is thus achieved, in a particularly advantageous manner, is that zones in which the medium to be squirted out remains, as might happen for example in the corners of the rectangular medium chamber, cannot form in the medium chamber. The diagonal throughflow of the medium chamber reliably avoids such deposits, which could ultimately also lead to closure of the medium chamber. Moreover, the diagonal throughflow of the medium chamber ensures that gas inclusions present in the medium are reliably flushed out. Of course, the medium chamber could also be squarexe2x80x94as seen in plan view.
According to a preferred exemplary embodiment, the medium chamber, the feeding duct, the squirting-out opening and the medium supply chamber are introduced from the (100) crystal plane or a plane of the silicon that is parallel thereto, so that the side walls for the said recesses are present at the abovementioned angle of 54.7xc2x0.
In order to achieve the diagonal throughflow of the medium chamber, the feeding duct and a connecting duct, which connects the squirting-out opening to the medium chamber, run at an angle of 45xc2x0 relative to a second crystal plane perpendicular to the (100) crystal plane, the longitudinal extent of the medium chamber then preferably being perpendicular to the said second crystal plane. As an alternative, however, it is also conceivable for the medium chamber to run at an angle of 45xc2x0 with respect to the said second crystal plane. The feeding duct and the connecting duct then run essentially perpendicularly to the crystal plane, so that in this case, too, it is again ensured that the medium chamber experiences diagonal throughflow.
As mentioned above, in a preferred embodiment, the diaphragm is formed by the bottom of the recess. In this case, the etching time or etching depth for producing this recess is dimensioned in such a way that the diaphragm preferably has a thickness of approximately 20 to 100 xcexcm.
In a preferred exemplary embodiment, the connecting duct has at least one partial duct perpendicular to the second crystal plane, the squirting-out opening being formed by the orifice of the said partial duct. In other words, a partial duct to the squirting-out opening then leads to the edge area of the printing chip, the said partial duct running perpendicularly to the said edge area, so that the medium emerges from the squirting-out opening preferably at an angle of 90xc2x0, as a result of which the drop can also be applied to the desired point on a substrate to be treated.
In a preferred embodiment, the feeding duct has a constricting point or is itself designed as a constriction. The flow rate or the volumetric flow from the medium supply chamber into the medium chamber can thus be set in a particularly simple manner. When the diaphragm is deflected, the constriction additionally prevents the medium from being forced from the medium chamber back into the supply chamber.
A particularly preferred exemplary embodiment is distinguished by the fact that the medium chamber and the supply chamber are located in one of the substrate parts and are delimited with respect to one another by a web, and that the feeding duct is formed in the other of the substrate parts and is located under the free end of the web. An overflow possibility for the medium from the supply chamber to the medium chamber is thus formed in a simple manner, in which case, moreover, the constricting point may be formed by the web.
Preference is attached to an exemplary embodiment of the printing chip in which a plurality of medium chambers, feeding ducts, connecting ducts and squirting-out openings are provided, so that the hot medium to be squirted out can be ejected simultaneously or in succession.
In a preferred exemplary embodiment, the printing chip has a heating element which heats the medium located in the supply chamber in order that it remains liquid and can be squirted out. In this case, it is preferred for at least one heating element to be fitted on the printing chip in such a way that it is located on the outer side of at least one of the substrate parts. As an alternative, it would also be conceivable for the medium that is to be squirted out to be introduced into the supply chamber such that it is already hot or preheated to such an extent that heating on the printing chip itself is not necessary. However, it is also possible to provide an external heating device, for example a halogen lamp, for heating the medium.
A preferred exemplary embodiment of the printing chip is distinguished by the fact that a temperature sensor is provided, which monitors the medium temperature, in particular. For this purpose, provision is made, in particular, of one temperature sensor located on the outer side on at least one of the substrate parts. Through the combination of temperature sensor and heating element, the printing chip can be kept at a temperature in such a way that the hot liquid medium in the printing chip neither undesirably solidifies nor is overheated, so that permanently good squirting-out results can be obtained.
Further refinements are specified in the subclaims.
The printing chip described above is used, in a preferred use, in particular, for squirting out hot liquid media whose temperature may reach far in excess of 1000xc2x0 C. The printing chip according to the invention affords advantages particularly in the case of liquid media whose melting point lies in this temperature range, so that metals, metal compounds or alloys and even glasses having a low melting point can be squirted out using the printing chip according to the invention.